Abstract: The resonance process of free electrons and external electric fields in noble metal (Ag) is described by using the polarization current difference equation. The difference equation is combined with the Maxwell equation. The finite-difference time-domain (FDTD) method is used. Based on the particle-in-cell (PIC) simulation software CHIPIC3D, the PIC simulation of the electron-excited surface plasmon polaritons (SPPs) is realized. Through simulating the SPPs excited by 100 keV electrons moving parallel to the surface of silver films, the field strength and mode of SPPs are observed and analyzed, and the simulation results are verified.